High

Spoofing via Deserialization of Untrusted Data in Microsoft Outlook

IdentifiersCVE-2026-21511CWE-502· Deserialization of Untrusted Data

CVE-2026-21511 is an Important-severity spoofing vulnerability in Microsoft Office Outlook. The provided content states that the issue is caused by deserialization of untrusted data and allows an unauthorized attacker to perform spoofing over a network. No further technical detail about the specific vulnerable code path, message format, or Outlook function is provided in the source material.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows an unauthenticated attacker to spoof data or identity over the network in the context of Microsoft Outlook. The supplied advisories also indicate spoofing impact may include potential NTLM credential disclosure in the broader February 2026 Outlook/Exchange spoofing class, but the provided content does not specifically confirm that outcome for CVE-2026-21511.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, reduce exposure by limiting untrusted content delivery paths to Outlook where feasible, increasing monitoring for suspicious Outlook-driven network authentication or spoofing activity, and hardening email and network controls to detect or block malicious content. Specific vendor-provided mitigations beyond patching are not available in the provided content.

Remediation

Patch, then assume compromise.

Apply the February 2026 Microsoft security updates for Microsoft Outlook and any affected Office/Microsoft 365 installations. Prioritize deployment through normal enterprise patch channels and verify successful installation across affected endpoints.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

Microsoft Corporation365 Appsapplication

Microsoft CorporationOfficeapplication

Microsoft CorporationOffice 2019application

Microsoft CorporationOffice 2021application

Microsoft CorporationOffice 2024application

Microsoft CorporationOffice Long Term Servicing Channelapplication

Microsoft CorporationOffice Macos 2021application

Microsoft CorporationOffice Macos 2024application

Microsoft CorporationOutlookapplication

Microsoft CorporationSharepoint Serverapplication

Microsoft CorporationSharepoint Server 2016application

Microsoft CorporationSharepoint Server 2019application

Microsoft CorporationWordapplication

Microsoft CorporationWord 2016application

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

11 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

11 SOURCESView more in app

flareio blogNews

Mar 13, 2026

Cybercrime Intelligence After February 2026 Patch Tuesday - Flare

A Microsoft Outlook client spoofing vulnerability that can be performed over a network.

nsfocus globalNews

Mar 4, 2026

Microsoft’s February Security Update of High-Risk Vulnerability Notice for Multiple Products - NSFOCUS, Inc., a global network and cyber security leader, protects enterprises and carriers from advanced cyber attacks.

A spoofing vulnerability affecting Microsoft Outlook.

cyber security newsNews

Feb 10, 2026

Microsoft Patch Tuesday February 2026 - 54 Vulnerabilities Fixed, Including 6 Zero-days

A spoofing vulnerability in Microsoft Outlook.

bleeping computerNews

Feb 10, 2026

Microsoft February 2026 Patch Tuesday fixes 6 zero-days, 58 flaws

A spoofing vulnerability in Microsoft Outlook (no additional details provided in the content).

+1 more in the timeline

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity4

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-21511

NVD

nvd.nist.gov/vuln/detail/CVE-2026-21511

MITRE CWE · CWE-502

Deserialization of Untrusted Data

Vendor Advisory

msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21511